Curing ovens are employed in semiconductor assembly for setting compounds such as epoxy resin and encapsulation molding compound that are introduced onto electronic components. These compounds are usually introduced onto electronic components in fluid form. They may also be suitable for reflowing. Based on the characteristics of these compounds, they may have to be heated according to specific heating profiles during the curing or reflowing process.
In particular, one implementation of curing ovens is in the curing of epoxy or reflowing of solder applied in the field of die bonding. Typically, semiconductor dice are bonded onto substrates such as leadframes using epoxy or solder as an adhesive. Epoxy is first introduced onto the substrate in fluid form at a bonding position, and a die is placed onto the epoxy at the bonding position. The epoxy or solder is then cured or reflowed by heating to solidify the bond.
Epoxy curing or reflowing using ovens is typically carried out according to specified heating profiles, such that the epoxy is exposed to various different temperatures during the curing or reflowing processes. FIG. 7 shows typical heating profiles for epoxy curing and reflowing processes, wherein the epoxy or solder should be controllably heated at varying temperatures. For epoxy curing, the epoxy may be preheated to a curing temperature, heated at the curing temperature for a specified period of time and then allowed to cool. For solder reflow, the solder may be preheated to a flux activation temperature, heated at the flux activation temperature for a specified period of time, then further heated to a reflow temperature whereat the heating temperature is maintained at the reflow temperature for a specified period of time. Thereafter, the solder is allowed to cool. The heating profiles may differ for different types of epoxy or solder.
One common feature of prior art curing ovens is that, if the epoxy or solder compound is to be heated at different temperatures, the curing ovens must have multiple thermal zones. Thus, curing ovens typically consist of multiple thermal zones wherein each zone is maintained at a single temperature. A substrate is heated according to a specified heating profile when it travels through the different thermal zones.
The use of curing ovens requiring multiple thermal zones to conduct such heating has several disadvantages. One disadvantage is that the space occupied by the curing oven is relatively large because of the need to have multiple heating zones. Its construction is also relatively complex, as different temperature zones have to be maintained and the substrate has to be conveyed through all the different temperature zones. Hence the cost of the curing oven is high. For curing oven applications where there is small-scale production and/or space limitations, such prior art curing ovens are not economical or cost-effective.
Moreover, due to the large size of such prior art curing ovens and their construction complexity, sealing of their enclosures is difficult. Thus, where nitrogen or forming gas is required in the oven to maintain a low level of oxygen content and prevent oxidation of the substrate, a large amount of such gas has to be continuously pumped to the curing oven to compensate for the leakage. Furthermore, the interaction among the interfaces of the different thermal zones induces instability on the substrate during the curing process. The final curing result may thereby be adversely affected.